UNITED STATES DEPARTMENT OF AGRICULTURE . FOREST SERVICE . FOREST PRODUCTS LABORATORY . MADISON, WI S

DETERMINATION OF MECHANICA L

PROPERTIES OF ADHESIVES FOR US E

IN THE DESIGN OF BONDED JOINT S

September 196 3

FPL-01 1

This Report is One of a Serie s

Issued in Cooperation with the

MIL-HDBK-23 WORKING GROUP ON COMPOSIT E

CONSTRUCTION FOR FLIGHT VEHICLE S

of the Departments of the

AIR FORCE, NAVY, AND COMMERC E
DETERMINATION OF MECHANICAL PROPERTIES O F

1 , 2

ADHESIVES FOR USE IN THE DESIGN OF BONDED JOINTS

B y

EDWARD W. KUENZI, Enginee r

a n d

GORDON H. STEVENS, Enginee r

Forest Products Laboratory, –

3 Forest Servic e

U.S. Department of Agricultur e

--- -

Summar y

This report describes methods for determining the basic mechanical propertie s

of adhesives subjected to shear or tension in bonds betweem relatively rigi d

adherends. The use of the adhesive properties for designing lap joints is als o

discussed .

Introductio n

The development of adhesives for the bonding of metals has made possible th e

design of efficient structural assemblies, and these adhesives are particularl y

suitable for joining thin sheet materials to produce lightweight aircraft parts.

Suitable design criteria for various types of joints and loading conditions ar e

1

–This research note replaces a report by the same title, published as Forest

Products Laboratory Report No. 1851 in 1956. It is another progress re -

port in the series (ANC-23, Item 54-1) prepared and distributed by the For -

est Products Laboratory under U.S . Navy, Bureau of Naval Weapons Nos.

19-61-8041 WEPS and 19-63-8022 WEPS and U.S. Air Force Nos. D O

33(616)61-06 and 33(657)63-358 . Results here reported are preliminary an d

may be revised as additional data become available .

2

–Trade names are used in this report at the request of the ANC-23 Panel, an d

their use implies no endorsement of the products named by the U.S. Forest

Products Laboratory or any other Government agency .

3

–Maintained at Madison, Wis., in cooperation with the University of Wisconsin .

FPL-01 1
lacking, largely because the behavior of the joints under various loads depends

on the mechanical properties of the adhesives in the joints, and such proper -

ties have not been determined.

Considerable work has been and is being done to determine the suitability o f

adhesives for bonding systems by conducting strength tests of lap joints.

Analyses of the strength data have been extended to determine “effective ”

elastic properties as well as the shear strength of various adhesives, and be -

cause of this, property values determined vary with the number of different

analyses employed. The “effective” elastic properties and strength value s

obtained are useful primarily for predicting the strength of lap-joint specimens . It would seem unlikely that design criteria based on a joint with considerable flexibility at failure (such flexibility as results from stressing the

adhesive far beyond the elastic range of behavior) would predict proper be -

havior at lower stresses, or for more rigid joints such as would be required

to bond skins to spars, edgings on sandwich, laps in sandwich facings, o r

doublers on sandwich facings.

It is the purpose of the work reported here to establish, if possible, methods

of test to determine the properties of adhesives when used in actual bonded

joints . Adhesive films or cast adhesives could have been evaluated, but sinc e

the surfaces, bonding conditions, and other factors could not be duplicated a s

when bonding metals, it was decided to evaluate actual bonded specimens.

The adhesive-bond properties that were to be determined were the shear modulus, modulus of elasticity, Poisson’s ratio , shear and tensile stress-strain

curves , and shear and tensile strength. These properties could then be used

in theoretical or empirical design criteria for any particular joint to be de -

signed. This report includes an illustration of the application of available de -

sign criteria for determining the behavior of lap joints.

Although there are few theoretical design criteria available for many types o f

bonded joints, the continued use of these joints will require such criteria, incorporating proper parameters, so that elastic behavior and deformations,

as well as impact and creep effects, can eventually be considered. Thus ,

ultimately, a joint could be designed by using actual adhesive properties rathe r

than purely by rule-of-thumb procedure .

Specimen s

For the accurate determination of the shear properties of most materials, the

torsion test of circular cylinders is unexcelled. For a solid cylinder, how -

ever, the shear stress varies radially fro m

at the perimeter , Thus, when loaded abov e

FPL-01 1 - 2 -

zero at the center to a maximu m

the proportional limit stress a t
the perimeter , the material is no longer homogeneous in a radial direction ,

and failing torque cannot be converted into shearing strength, just as failin g

bending moment in a beam cannot readily be converted into actual tensile o r

compressive stresses . Most of this difficulty can be overcome by twisting a

tubular section . The thinner the tube wall, the more nearly the perimete r

shear stress approaches the stress at the inner diameter, and nearly pur e

shear strength , as well as the entire shear stress-strain curve, can be ob -

tained .

Specimens for the determination of the shear and tensile properties of adhe -

sives were made by bonding together the ends of 61S aluminum alloy tubing

1-1/2 inches in inside diameter and 2 inches in outside diameter. Where mor e

than one glue line was used, “washers ” of the tubing, each 1/8 inch thick,

were bonded to each other and between two 2-1/2-inch-long tubes (figs. 1 an d

2 ). This resulted in a specimen with several adhesive bonds that had a tota l

deformation several times that of a single bond, and a more accurate deter -

mination of bond deformation was possible .

Each end of the tubular specimen was threaded or slotted for mounting in the

torsion machine . Specimens with the threaded ends could also be evaluate d

in tension . Specimens of Metlbond 408 and FM 1000 adhesive had the ends o f

the specimens slotted and were loaded in compression, since they were no t

suitable for tension loading, to determine the modulus of elasticity. In figur e

3 a slotted specimen is shown partially in place in the torsion machine .

Bonding of Test Specimens –

4

The ends of the tubes and “washers” were smoothly surfaced in a lathe an d

then etched and bonded with the following adhesives:

Redux K-6, a high-temperature-setting, two-component formu -

lation of phenol resin solution and a vinyl polymer powder .

Scotchweld AF-6, a high-temperature-setting adhesive formu -

lation of acrylo-nitrile-butadiene rubber and phenol resin in th e

form of an unsupported tape .

Metlbond MN3C Nylon Tape , a high-temperature-setting formu -

lation of neoprene, nylon , and phenol resins supported as a fil m

on nylon-fabric tape .

4

–Bonding procedures were devised and carried out under the supervision o f

H. W. Eickner of the U.S. Forest Products Laboratory .

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Metlbond 4021, a high-temperature-setting formulation of nitril e

rubber and phenol resin in the form of an unsupported film.

FM 47 Tape, a high-temperature-setting formulation of vinylphenolic type supported on woven glass fabric .

Metlbond 408, a high-temperatur e -setting modified epoxy resin

in an unsupported film .

FM 1000, a high-temperature-setting unsupported epoxy poly -

amide film .

Epon 422J Tape , a high-temperature-setting formulation of the

epoxy-phenol type supported on woven glass fabric .

Epon VIII, a formulation of epoxy resins.

Tube and washer ends that were to be bonded with the first seven of the abov e

adhesives were etched for 10 minutes at 140° to 16 0

0 F. in a solution consisting of 10 parts by weight of concentrated sulfuric acid, 1 part of sodium bichromate , and 30 parts of water . Specimen ends that were to be bonded with

Epon VIII and Epon 422J adhesives were etched for 20 minutes at 140° t o

1 6 0

0 F. in a solution consisting of 12.5 parts of concentrated sulfuric acid,

3.5 parts of sodium bichromate, and 30 parts of water. The etching solution

was rinsed from all specimen parts with running cold and hot water . T he

parts were then dried in a current of air .

After the parts were ready for bonding, they were carefully assembled without adhesive , and total length was measured with a micrometer reading t o

0.001 inch. This was done so that the total adhesive thickness could be obtained by subtracting the original length from the length of the bonded specimen .

The specimens were then assembled with the adhesives. The parts wer e

alined in a straight piece of angle iron trough, and pressure was applied with

a calibrated spring compressed by a bolt that passed through the tubes. Pres -

sure was controlled by tightening the bolt until the spring was compressed t o

the deflection necessary to produce the desired pressure. The bonds wer e

then cured by placing the entire assembly in an oven at the proper curing

temperature . The particular procedure and curing temperature and pressur e

for each adhesive are given as follows:

Redux K-6 . --One medium coat of the liquid component of the adhesive was brushed on the surfaces that were to be bonded, and

the powdered component was sprinkled immediately onto the we t

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coating . Excess powder was brushed from the surface. The

adhesive was air dried overnight, and the specimen was then

cured for 15 minutes at a temperature of 300° F. and a pressur e

of 100 pounds per square inch.

Scotchweld AF-6 . --A single film of this tape adhesive was used

at each joint in a specimen. The film was cut into washer s

slightly larger in outside diameter and smaller in inside diamete r

than the tubular specimen. After the specimen was assembled,

the adhesive was cured for 45 minutes at a temperature of 325 °

F. and a pressure of 150 pounds per square inch.

Metlbond MN3C Nylon Tape . --The surfaces that were to be bonded

were sprayed with four coats of a priming component to a tota l

thickness of 0.001 to 0.002 inch. Each coat was air dried fo r

30 minutes, and the final coat was dried for 3 to 5 hours. The

specimen was assembled with a single film of tape adhesive, in

the form of oversize washers, at each joint. The curing wa s

completed at a pressure of 50 pounds per square inch for 12 minutes at 300° F. and 30 minutes at 335° F.

Metlbond 4021 . --A single coat of the priming component wa s

sprayed on each of the surfaces to be bonded and air dried fo r

18 hours. The specimen was then assembled with a single fil m

of adhesive, in the form of an oversized washer, at each joint.

The assembly was then clamped with a pressure of 100 pounds

per square inch and cured for 1 hour at 350° F.

FM 47 Tape . --A single coat of the priming component wa s

sprayed on each of the surfaces to be bonded and precured fo r

1 hour at 220° F. The specimen was then assembled with a

single film of the tape adhesive, in the form of an oversized

washer , at each joint. The adhesive was cured at a pressure o f

200 pounds per square inch for 30 minutes at a temperature o f

300° F .

Epon 422J Tape . --A specimen was assembled with a single fil m

of the tape adhesive , in the form of an oversized washer, a t

each joint. The adhesive was then cured at a pressure of 15

pounds per square inch for 30 minutes at a temperature of 330 °

F .

Metlbond 408 . --A specimen was assembled with a single fil m

of the tape adhesive , in the form of an oversized washer, a t

FPL-01 1 - 5 -
each joint. The adhesive was then cured at a pressure of 1 5

pounds per square inch for 60 minutes at a temperature of 350 °

F .

FM 1000 . --A specimen was assembled with a single film of the

tape adhesive , in the form of an oversized washer, at each

joint. The adhesive was then cured at a pressure of 15 pounds

per square inch for 60 minutes at a temperature of 350° F.

Epon VIII. --One medium coat of adhesive was applied to each

surface that was to be bonded. The adhesive was cured fo r

90 minutes at a temperature of 200° F. and a pressure of 1 0

pounds per square inch.

After the bonded specimens cooled, excess adhesive was carefully removed

from their exterior and interior surfaces.

Testing Procedure s

The bonded specimens were tested either in torsion to determine shear stressstrain curves, shear modulus, proportional limit stress, and shear strength,

or in tension to determine modulus of elasticity, proportional limit stress,

and tensile strength. In several instances, torsion specimens were first

tested in compression or tension at a load within the elastic range, to obtain

modulus of elasticity values and then tested in torsion.

Torsion Test s

Loading bars of the torsion testing machine with threaded studs were screwed

into the test specimens with threaded ends. Difficulty in unscrewing tested

specimens with threaded ends prompted design of specimens with slotted ends.

Collars with crossed bars were fitted to the torsion machine for twisting

specimens with slotted ends. Figure 3 shows a specimen with slotted ends

partially in place in the torsion machine . The test specimen was fitted with a

detrusion gage to measure the total twist over a 1-1/2 inch length of the specimen that included the adhesive bonds. The gage , shown on the specimen in

figure 1, consisted of a ring held to the specimen with three pointed set screws

and a Tuckerman optical strain gage fastened to a bar rigidly attached to the

ring . The position of the ring was adjusted so that the ring was concentri c

with the tubular test specimen, and the movable knife edge of the gage wa s

just in contact with the specimen surface . This arrangement made it possibl e

to read total detrusions to 0.000001 inch by reading the vernier scale in the

FPL-01 1 - 6 -
Tuckerman autocollimater . For a specimen with an outside diameter o f

2 inches the detrusion readings were directly in radians. After the threade d

specimens with the detrusion gage were mounted in the torsion machine, a n

initial load, not exceeding the elastic range of the adhesive, was applied an d

then removed from the specimen . This first application of load tightened the

threaded specimens onto the threaded studs of the torsion machine and brough t

the specimen ends into firm contact with a shoulder at the base of the threaded

stud .

When the load was reapplied to the specimen, detrusion values were read a t

various loads to determine the data for the stress-strain curve . Figure 4

shows the effect of the preloading on the stress-strain curve for a specimen .

After the initial loading, the stress-strain curve was about the same shap e

for a second and third loading even after waiting for 1 hour before application

of the third loading. The same test procedure was used for the specimen s

with slotted specimens to be consistent on loads applied to specimens. T h e

data from the second loading were used in determining the adhesive proper -

ties .

To determine the detrusion in the adhesive bonds, it was necessary to subtrac t

the detrusion in the aluminum tube from the total measured detrusion. Th e

amount of detrusion in the aluminum was determined by testing a tube withou t

bonds and obtaining a shear modulus. This shear modulus was then used t o

calculate aluminum detrusions for the bonded specimens. Before they wer e

tested, these specimens without bonds were subjected to the heating schedul e

used to cure the bonded specimens.

Shear stresses were calculated from the formula: –

5

where : shear stres s

outside diameter of the tub e

inside diameter of the tub e

applied torque .

(1)

5

–This formula gives a surface shear stress of 0.933T for a tube of the siz e

tested (2-inch outside diameter, 1-1/2-inch inside diameter). The stres s

at the inner diameter is 0.699T .

FPL-01 1 - 7 -
For the specimen without bonds, the shear modulus was calculated from th e

formula :

(2)

where :

The test of the

shear modulus of aluminu m

shear stres s

length of the tube between measuring points of the detrusion

gag e

detrusion measured by the gage .

tube without bonds gave a shear modulus value of 3,700,00 0

pounds per square inch . This value was used in calculating the total adhesiv e

bond detrusion from the formula :

(3)

where : adhesive bond detrusio n

detrusion measured by gag e

shear stres s

shear modulus of aluminu m

length of tube between measuring points of the detrusion gag e

total thickness of adhesive between measuring points of the

detrusion gage .

The shear strain in the adhesive was obtained by calculating the value o f

(4)

Stress-strain curves were then drawn and the shear modulus, the initia l

slope of the stress -strain curve, was given by the formula :

( 5 )

FPL-01 1 - 8 -
Tension or Compression Tests

Tension tests were made only on the threaded specimens. The torsion loadin g

bars were screwed into the ends of the specimen and suspended by bolts i n

spherical bearings in the testing machine . The specimens with the slotte d

ends were loaded in compression to determine a measured modulus of elasticity . These specimens were loaded while supported on a base block havin g

a spherical seat . Two Tuckerman optical strain gages of 1-1/4-inch gag e

length were used to measure the total deformation across the bonded parts.

The testing apparatus is shown in figure 2 . Deformation readings were take n

at various applied loads to determine data for load-deformation curves.

In order to determine deformation in the adhesive bonds, it was necessary t o

subtract the deformation in the aluminum tube from the total measured de -

formation . The amount of deformation in the aluminum was determined b y

testing a tube without bonds to obtain a modulus of elasticity, which was then

used to calculate aluminum deformations for the bonded specimens.

6

Tensile stresses were calculated from the formula:

where : tensile stress or compressive stres s

outside diameter of tub e

inside diameter of tub e

applied tensile load .

(6)

For the specimen without bonds, the modulus of elasticity was calculated fro m

the formula :

(7)

where : modulus of elasticity of aluminu m

tensile stress or compressive stres s

length of the tube between measuring points of the deformatio n

gag e

total deformation measured by the gage .

6

–This formula gives a stress of 0.726P for the size of tube tested (2-inch outside diameter, 1-1/2-inch inside diameter) .

FPL-01 1 - 9 -
The test of the tube without bonds gave a modulus o f

1 03 500,000 pounds per square inch. This value wa s

adhesive deformation from the formula :

(8)

where : adhesive bond deformation

deformation measured by the gag e

tensile stress or compressive stres s

modulus of elasticity of aluminu m

length of the tube between measuring

gag e

elasticity value o f

used to calculate the tota l

points of the deformation

total thickness of adhesive between measuring points of the de -

formation gage .

The tensile (or compressive) strain in the adhesive was obtained by calculating

the value of

(9)

A “measured” modulus of elasticity of the adhesive, the initial slope o f

the stress-strain curve, was calculated by :

(10 )

Because the adhesive bonds are very thin and less rigid than the adherends,

these bonds must be restrained. Formula (10), therefore, does not give a

correct modulus of elasticity for the adhesive bond. Normally, in most tests

of materials, the specimens are proportioned so that natural lateral contrac -

tion (Poisson’s ratio effects) can occur in tension, or lateral expansion can

occur in compression. The following analysis is presented as a method of obtaining the correct modulus of elasticity, It is assumed in the analysis

that the bond is isotropic and that the lateral contractions or expansions of the

bond must be the same as those of the adherends under the same tensile stress.

FPL-01 1 -10 -
(11 )

where :

The relation between a stress, and the resultant strains for an isotropi c

material is given by the formula 7 .

stress in direction x

strains in directions x , y , and z

modulus of elasticit y

Poisson’s ratio .

Rewriting this formula for the adhesive bond gives:

(12 )

Assuming that the lateral strains in the adhesive are the same as those in th e

adherends, then :

(13 )

where : -- Poisson’s rati o

- - adherend strai n

for adheren d

in direction x .

Substituting equation (13) into equation (12) gives:

o r

(14 )

7

–Timoshenko, S . Theory of Elasticity. McGraw-Hill. New York, N. Y .

p. 11. 1934 .

FPL-01 1 -11 -
the “measured” modulus of elasticity, and since the stress in

the adhesive is the same as that in the adherend 

where E is the modulus of elasticity of the adherend. Substituting these ex -

pressions into equation (15) gives:

(16 )

Thus equation (16) relate s but contains an unknown Poisson’s

rati o For isotropic materiials it is known that:

where: shear modulus of the adhesive bond.

Substituting equation (17) in equation (16) and solving

(17 )

for gives:

(18 )

Then with a shear modulus, , from a torsion

to find the Poisson’ s ratio, of the adhesive

Poisson’s ratio can then be used along with the

modulus of elasticity, from the formula :

test, formula (18) can be used

bond. Finally, this value o f

shear modulus to find the

(19 )

For the computations in this report, the values of the adherend propertie s

were E = 10, 500, 000 pounds per square inch and v = 0.42. Actually, for -

mula (18) is relatively insensitive to the values of E and v because they appea r

as small terms in part of the denominator .

FPL-01 1 -12 -
Test Results

The results of tests to determine the adhesive bond properties are given in

table 1 and summarized in table 2 . Complete information was not obtained

for all the adhesives and the data should not be construed as representing av -

erage property values.

The Poisson’s ratio varied from 0.294 for the Epon 422J to 0.498 for the

Metlbond MN3C . Scotchweld AF-6, Metlbond MN3C, and Metlbond 4021 wer e

the least rigid of the adhesives tested. The shear strength of these adhesive s

was about 2,500 pounds per square inch for the Metlbond MN3C, about 3,90 0

pounds per square inch for the Scotchweld AF-6, and about 4,600 pounds pe r

square inch for the Metlbond 4021 . The shear strength values for the remaining, more rigid, adhesives ranged from about 3,700 to 8,200 pounds pe r

square inch. A summary of the average values for the properties of the adhesives is presented in table 2 .

Shear stress-strain curves for the adhesive bonds are given in figure 5. Thes e

curves were drawn from data for the specimen within a group that had propert y

values near the group average . The big difference in initial slope of thes e

curves demonstrates the difference in rigidity . The area under the stressstrain curves represents work done in shearing the specimens. This work can

be used as an indication of behavior under impact: the larger the amount o f

work, the greater the impact resistance . The work to. proportional limi t

stress , or modulus of resilience, can be calculated from the formula :

(20 )

The work to failure, W , was obtained by measuring the entire area under the

curve with a planimeter . Values of work to proportional limit and work t o

failure for the adhesives are :

Adhesiv e Work to proportiona l Work to failur e

limi t

In. -lb. per cu. in. In. -lb. per cu. in.

Redux K- 6 7 5 9,10 0

FM 4 7 3 5 1 3 0

Epon 422 J 3 5 2 1 0

Epon VIII 5 5 6 7 0

FPL-01 1 -13 -
Adhesiv e Work to proportiona l Work to failur e

limi t

In. -lb. per cu. in . In. -lb. per cu. in .

FM 100 0 1 2 0 16,000 +

Metlbond 40 8 3 5 12,00 0

Scotchweld AF- 6 1,50 0 1,80 0

Metlbond 402 1 5,20 0 5,20 0

Metlbond MN3 C 2,20 0 2,20 0

Thus the less rigid adhesives (Scotchweld AF-6, Metlbond 4021, and Metlbon d

MN3C) had much greater resilience in elastic behavior than the more rigi d

ones, but the work to failure was greatest for FM 1000, a rigid adhesive, an d

least for FM 47, another rigid adhesive .

Desire of Lap Joint s

A joint commonly used for sheet materials is the adhesive-bonded lap joint.

Theoretical analyses–8 have been developed for the stresses that occur in the

adhesive bond in lap joints between thin members subjected to tension . T h e

property values determined for a rigid and a resilient adhesive were used i n

the following design of lap joints between 0.064-inch aluminum sheets.

Since the theories developed are based on elastic behavior of materials, th e

analysis performed here does not apply to stresses exceeding proportiona l

limit values.

It is of interest to examine the stress variation in a lap joint. The shea r

stress at any point a distance x from the center of a lap joint that is free to

bend is given by the formula: –

8

(21 )

8

–Goland, M. and Reissner, E. Stresses in Cemented Joints. Journal o f

Applied Mechanics. 1944 .

FPL-01 1 -14 -
where : shear stress at point x—

load on the lap joint, per unit width

length of the overlap

thickness of each bonded shee t

distance from the center of the overlap, measured in the direc -

tion of lap

shear modulus of the adhesive bond

thickness of the adhesive bond

modulus of elasticity of the bonded sheets

Poisson’s ratio for the bonded sheets.

The tearing (tensile) stress at any point a distance x from the center of a lap

joint that is free to bend is given by the formula: –

8

(22 )

where : -- tensile stress at point x

- - modulus of elasticity of the adhesive bond

FPL-01 1 -15 -
Shear and tensile stresses were computed by using formulas (21) and (22) fo r

Redux K-6 (for which G = 180, 000 pounds per square inch, t = 0.003 inch,

g g

and E = 500,000 pounds per square inch) and Metlbond MN3C tape (for which

g

G

g = 1,530 pounds per square inch, t = 0.007 inch,

g

per square inch). The bonds were 1/2-inch lap joints

sheets of aluminum (for which E = 10,000,000 pound s

v = 0.3) under a load of 300 pounds per inch of width.

and E = 5,200 pounds

g

of 0.064-inch-thic k

per square inch an d

Stresses produced in the rigid bond of Redux K-6 adhesive are shown in fig -

ure 6 . The shear stress is rather low throughout the central portion of th e

lap and rises quite sharply to a maximum at the end of the lap. This maximu m

shear stress is 3,520 pounds per square inch, which is about six times th e

average shear stress of 600 pounds per square inch . The normal stress is

also low, even compressive in nature throughout the central portion of the lap ,

and rises sharply to a tension of about 4,400 pounds per square inch at the end

of the lap.

Stresses produced in the resilient bond of Metlbond MN3C tape adhesive ar e

shown in figure 7. The shear stress is nearly constant throughout the lap; it

rises gradually to a maximum value of 630 pounds per square inch at the end

of the lap, which is only 5 percent greater than the average shear stress. T h e

normal stress varies from compression at the center of the lap to a tension o f

about 260 pounds per square inch at the end of the lap, but the rise to maximum tension is much more gradual than that for the rigid Redux K-6 adhesive .

A comparison of the shear deformation in the bonds of Redux K-6 and Metlbon d

MN3C at a load of 300 pounds on a 1/2-inch lap of 0.064-inch aluminum sheets

is shown in figure 8. In this figure , the deformations shown are drawn to th e

same scale for each adhesive . The more resilient Metlbond MN3C bond de -

forms about 48 times as much as the rigid Redux K-6 bond. The Metlbon d

MN3C adhesive bond was 2-1/3 times as thick as the Redux K-6 adhesiv e

bond . The deformation of the Metlbond was therefore 2-1/3 times greate r

than if it had been of the same thickness as the Redux bond.

FPL-01 1 -16 -
Formulas (21) and (22) give shear and normal stresses as functions of th e

distanc e x . These formulas , when written for the maximum shear and tearin g

stresses, become :

(23 )

a n d

These formulas ((23 ) and (24)) were used to determine the loads at which the

(24 )

maximum shear and tensile stresses, T and u reached proportiona l

m a x max ‘

limit values for various amounts of overlap of 0.064-inch aluminum allo y

sheets . Loads were calculated for Redux K-6, a rigid adhesive, and fo r

Metlbond MN3C tape, a more resilient adhesive. The calculated loads coul d

be considered as design loads for the particular amount of overlap, becaus e

greater loads would probably produce permanent deformations. The resultan t

loads are plotted against the overlap length in figure 9. The curves show tha t

for the resilient adhesive Metlbond MN3C shearing stress is the controllin g

stress in the design of a lap joint for all lengths. The controlling stress i n

lap joints bonded with Redux K-6, a rigid adhesive , may be either shearing o r

tensile stresses . Shearing stress is the controlling stress for very shor t

overlap, tension stress is controlling for the overlap of moderate length a s

usually used , and shearing stress is again the controlling stress for ver y

long overlaps,

The design curves also show that the more resilient Metlbond MN3C tape ca n

have considerably higher design loads than can the more rigid adhesive ,

Redux K-6 . This, of course , would be expected , because the more rigid th e

adhesive the greater the stress concentration at the end of the lap. Th e

curves in figures 6 and 7 show the stress distribution over the length of a lap

joint . It should be recalled, however, that the deformation occurring i n

joints made with the resilient adhesive, Metlbond MN3C would be muc h

greater than that in joints made with Redux K-6 (fig. 8). The effects of join t

deformation on the structure must be considered and may be more importan t

than load-carrying capacity .

FPL-01 1 -17 -
The analysis and application of adhesive properties to the design of bonded

lap joints is theoretical. Design curves presented in figure 9 are based on the

adhesive properties only within the elastic range . If even a small portion of

these curves could be checked by actual test there would be some basis fo r

using these formulas in the design of lap joints.

FPL-01 1 -18 - 1.-3 3




FPL-011
Figure 1. -- Torsion test specimen with Tuckerman gage and holder for measuring total

detrusion across bonds.

ZM 96917 F
Figure 2. --Tension test specimen with Tuckerman gages for measuring

total detrusion across bonds.

ZM 96918 F